1. Field of the Invention
This invention relates to hydroelectric power-generating devices that use underwater currents to drive electricity-generating turbines.
2. Description of the Prior Art
The use of underwater power generators for generating electricity from water current flow, such as rivers and oceans, is known in the art. There are two types of prior devices: stationary turbines and tethered turbines. Stationary types of turbines are comprised of stationary towers based on the ocean floor. Electricity generating turbines are mounted on the towers at a fixed depth, with turbine rotor blades facing the flow of an ocean current. The disadvantages of this type of design are the underwater construction costs, the inability to service the turbines on the surface, and fact that the depth of the turbines cannot be changed to adjust for ocean currents that are not constant year round at the fixed depth. An example of this type of device is a 300 kW current turbine under development by the British having 20-meter diameter rotor blades. The turbines are mounted at a fixed depth on towers to be deployed in currents around the British Isles.
Tethered devices are designed to operate underwater, and are kept in place by a tether that is anchored to the ocean floor. In some cases, a wing (hydrofoil) provides lift and /or ballast tanks provide buoyancy to keep the devices from descending. Some devices use a buoyancy chamber to regulate their overall buoyancy thereby adjusting their operating depths in a current stream. Other devices add a moveable wing that serves as a stabilizer to control the depth of the device. The wing is adjusted to cause the device to dive or surface in response to emergency conditions such as floating debris on a river.
An example of this type of turbine device is an underwater power plant called the Coriolis Project. The design called for a ducted, catenary turbine, 171 meters in diameter, capable of producing 83 MW of electricity. The turbine was to be tethered at a fixed depth.
What is needed is a way of controlling a tethered, underwater, water current-driven turbine, power-generating device. In order to keep installation and maintenance costs of such a device down, it is desirable to provide minimal underwater structures and construction. This means that one must be able to safely bring the device to the surface for maintenance or for replacement of single or multiple turbines without altering the arrays, consisting of a plurality of turbine modules.
Since recent studies have shown that underwater ocean current speeds vary seasonally at a given depth, programmable controls are desirable for the precise adjustment of operating depth. The turbine modules' lift and buoyancy should be adjusted periodically at different times of the year to compensate for predictable seasonal current velocity changes. The depth should also be adjustable as required to compensate for unpredictable short-term underwater current changes.
It is also desirable to provide for emergencies that require a complete shutdown of the device, with or without immediate surfacing of the device for maintenance or repair.